Communications: radio wave antennas – Antennas – With means for moving directive antenna for scanning,...
Reexamination Certificate
1999-05-20
2001-03-20
Le, Hoanganh (Department: 2821)
Communications: radio wave antennas
Antennas
With means for moving directive antenna for scanning,...
C343S7810CA, C343S757000
Reexamination Certificate
active
06204822
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the field of antenna systems for communicating between an earth station and a satellite. More particularly, it relates to a reflector for use in an antenna system and to an antenna system using a plurality of such reflectors for broadband satellite communications systems operating in the microwave and millimeter wave frequency bands. The invention further relates to antennas for communicating with low- and mid-earth orbiting satellites, which traverse the sky somewhat rapidly.
BACKGROUND OF THE INVENTION
In past decades, there have come into widespread use communications systems which employ earth-orbiting satellites to relay communications between earth-based stations (“satellite communications systems”). Now under development and in early stages of deployment are satellite communications systems which utilize broadband signaling and operate in the 11-14 GHz (Ku) band, the 20-30 GHz (Ka) band, and higher millimeter wave bands between about 30 and 70 GHz. (Such systems are hereafter referred to as MMW systems or as Ku, Ka or V band systems.) Many of these MMW systems employ low-earth-orbit (LEO) or mid-earth-orbit (MEO) satellites in constellations, to provide bi-directional, high-speed data links from and to customer premises equipment (CPE) located at various places. They may also use one or more geostationary satellites (GEO's) in combination with LEO or MEO satellites in some types and modes of communications. The Spaceway, Expressway, Cyberstar, and Teledesic systems are the most well-known of these new systems; but in fact there are more than twenty MMW satellite systems in various stages of development and implementation.
All of these MMW systems are global in that they facilitate communications with CPE locations at virtually all points on the surface of the earth. End-user applications for these networks include placing CPE at various types of customer locations, including, for example, large and small business locations, telephone company points of presence (POP), multi-tenant office and apartment buildings, public telephone installations, and individual residences. One significant requirement for direct access to the satellite system from these locations is that a CPE antenna must be provided and that antenna must be capable of tracking simultaneously at least two LEO or MEO satellites to maintain a connection between the user and the network. In many cases, the antenna also must be able to track a geostationary satellite, as well. Further, it is important that such CPE antennas be low-cost devices capable of being made in a high-volume production environment.
At the customer location, the CPE antennas for these new MMW systems must acquire and track multiple satellites. In typical operations, a first LEO satellite is tracked across the sky from its acquisition horizon in the South or North, until it nears the opposite horizon. At that time, a second LEO satellite is acquired as it rises from its acquisition horizon. For a short time, both satellites are tracked, until the CPE link is handed off from the first satellite to the second satellite. This process is repeated as the various LEO or MEO satellites traverse the sky at each CPE location. In order to maintain operative communications links with the satellites of these MMW systems, the antennas used at the customer sites must be able to track at substantially any local azimuth angle and at all elevation angles above about fifteen (15) degrees. (Operation at lower elevation angles is not practical due to the increased atmospheric path loss and the presence of trees and buildings or other nearby structures.) This requires an antenna system with a full sky positioner mechanism that has sufficient accuracy to maintain tracking of a satellite such that a narrow, “pencil” transmission beam stays centered on the satellite as it moves across the sky. To assure adequate noise margin in the communications links, the CPE antennas are required to have diameters ranging from about 0.4 meters for “low end” (i.e., least expensive) residential units to “typical” diameters of about 0.75, 1.0 and 1.5 m for business CPE units. At these diameters, the beam widths at Ku, Ka band and V band are fractions of a degree; therefore, the positioner mechanism for the antenna must be capable of pointing the antenna to better than 0.1 degree of the target satellite position, at all points in the sky. This requires relatively precise positioners, and they often must move not only the entire MMW antenna but also its associated transmitter and receiver electronics.
Both electrically scanned and mechanically scanned antennas previously have been built for operation in satellite communication systems at the lower Ku and C band satellite communication (SATCOM) frequencies. In particular, the industry has a long history of providing Very Small Aperture Terminals (VSAT) for use in C and Ku band satellite links from customer premises. Such existing VSAT terminals use fixed mounted antennas that typically point to one specific GEO satellite location and do not need to be scanned or moved during operation. This fixed VSAT antenna approach is much easier to implement than the approach needed for the new MMW systems that must constantly track moving satellites across the sky.
Traditional parabolic reflectors, although easily manufactured in the small sizes needed, cannot be scanned readily with electronic means. Instead, they must be moved physically to point directly towards the orbiting satellite as it moves across the sky. In these MMW systems, this requires that the parabolic antenna be capable of being pointed to nearly all points in the entire hemisphere above some minimum elevation angle with respect to the horizon. Using traditional parabolic reflector antennas as are now used in Ku and C band VSAT antenna systems, in order to have multiple beams (to communicate simultaneously with multiple satellites), a separate reflector antenna is required for each beam, each having its own mechanical positioner mechanism. Multiple antennas must be used to facilitate the simultaneous tracking of two MEO or LEO satellites that are typically at opposite directions in the sky, and in the case where a GEO satellite is also employed, as many as three separate full-motion antennas must be employed simultaneously. Most of these antennas will be enclosed by a radome, both for aesthetic reasons and to avoid the effects of the environment (e.g., wind, ice, snow, insulation, etc.) on the tracking accuracy of the antenna system.
At the installation site for two (or three) relatively large and complex parabolic reflector tracking antennas, these antennas would be required to be spaced by about 5 m from each other so as not to interfere with each other during concurrent operation. Each antenna would be required to employ a relatively expensive and complicated mechanism to move the entire antenna to point towards the location of each tracked satellite as it moves in the sky. This relatively large installation will present an aesthetic and logistical problem at many locations. Since each antenna not only is moving constantly but also is transmitting RF energy, provisions for radomes must be made to avoid wind deflections and to make the installation safe from and for children, pets, etc.—particularly in residential and business sites. It is also not clear that a suitable high-accuracy positioner can be manufactured at a reasonable, low cost.
One might think of electrically scanned antennas as an alternative, but at the large scan angles required (i.e., nearly a full hemisphere coverage) in these new MMW systems, electrically scanned solid state phased arrays become difficult to implement. Moreover, they would have to have hundreds or even thousands of individual elements, making them both difficult to manufacture and quite costly. It is, however, technically feasible that monolithic GaAs semiconductor integrated circuits could provide 20-50-mW per transmit/receive element; and when used in groupings of around one hundred
Cardiasmenos Apostle G.
Monk Anthony D.
Rhoades Luther E.
Sangiolo John
L-3 Communications/Essco, Inc.
Le Hoang-anh
Wolf Greenfield & Sacks P.C.
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